PbSe Nanowires in Non-Coordinating Solvent

ABSTRACT

A PbSe nanowire having an aspect ratio of about 100:1 and having a diameter of less than 20 nm. A PbSe nanowire produced by the process comprising reacting PbO with oleic acid in 1-octadecene or other non-coordinating solvent and producing Pb oleate in a flask, heating the Pb oleate to between 225 and 275 C under inert gas, injecting a first solution of Se dissolved in trialkylphosphine into the flask and producing a second solution, heating the second solution, maintaining the temperature &gt;200 C in the flask, and resulting in PbSe nanowires.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Non-Provisional application claiming priority toand the benefits of U.S. Provisional Application No. 61/355,260 filed onJun. 16, 2010, and U.S. patent application Ser. No. 13/087,642, filed onApr. 15, 2011, entitled Method for the Formation of PbSe Nanowires inNon-Coordinating Solvent, the entirety of each is herein incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This disclosure concerns a method for the synthesis of PbSe nanowires ina non-coordinating solvent system.

2. Description of Related Art

One-dimensional nanowires possess many device fabrication advantagesover their zero-dimensional nanocrystal counterparts. Primarily, theseadvantages stem from the potential to access the effects of quantumconfinement in a structure with one-dimension that extends into themacroscopic regime. This combination of properties can ease difficultiesassociated with interfacing to a given material, yet still permittransport of electrons through a continuous inorganic structure withoutrelying on hopping or tunneling to adjacent particles. Realization ofthis concept would be of substantial importance to many electronicapplications. A particularly compelling material to examine for thispurpose is PbSe, a narrow band gap semiconductor of interest for bothphotovoltaic and thermoelectric applications due to its large excitonBohr radius and ability to absorb and emit infrared photons. Thismaterial has received significant attention as a potential candidate forexploiting multiexciton generation (MEG) in photovoltaic cells, and highaspect ratio structures may lead to improvements in these devices byreducing the rate of multiexciton Auger recombination.

Various methods have been reported for the formation of one-dimensionalnanostructures, with the bulk of these relying on either electrochemicaldeposition in a pre-formed template or Vapor-Liquid-Solid (VLS) growthfrom a catalyst droplet on a solid support. The synthesis of nanowiresin solution, analogous to the preparation of colloidal nanocrystals, hasbeen less widely examined despite several potential advantages to theapproach. This “bottom up” solution synthesis offers both the potentialfor smaller diameter structures, as well as scalability to obtain largerquantities of material, both important features for their eventualapplication. In addition, the organic ligands encapsulating thesematerials facilitate their manipulation in a variety of solvents,expanding possibilities for self-assembly and low cost deviceprocessing.

Several methods for the solution preparation of PbSe nanowires have beenreported in the literature, but due to issues related to theirreproducibility, dimensions, or purification, are not ideal forincorporation into a photovoltaic device. For example, the procedurereported by Lifshitz, et al. employs a KBH₄ reduction that generates KCland oxidized B salts, while Hull et al. utilize a Bi-Au catalystparticle to encourage one dimensional growth. These species representadditional components that must be separated from the product prior toutilization in a device as they could potentially interfere withelectron transport. The procedure of Cho et. al. is more straightforwardfrom an isolation standpoint in that the nanowire growth is reported tooccur through an oriented attachment of PbSe nanocrystals, but inpractice can be difficult to control due to the large volume of reagentthat must be rapidly added to the reaction. Additionally, all three ofthese prior procedures are conducted at a relatively high dilution,increasing the volume of solvent required as the reaction is scaled up.

BRIEF SUMMARY OF THE INVENTION

This disclosure concerns a method of making nanowires in a single flaskand in a non-coordinating solvent involving the reaction of PbO witholeic acid to produce Pb oleate, heating the Pb oleate to a preferredtemperature with additional coordinating ligands, injecting a solutionof Se to produce a second solution, heating the second solution, andmaintaining the temperature, resulting in nucleation and growth of PbSenanowires.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows Scanning Electron Microscope (SEM) images of PbSe nanowirematerial produced in non-coordinating solvent in the presence oftetradecylphosphonic acid.

DETAILED DESCRIPTION OF THE INVENTION

The purpose of this invention is a method for the synthesis of PbSenanowires in a non-coordinating solvent system.

These PbSe nanowires are produced using a single-flask solution phasesynthesis reaction employing a non-coordinating solvent. While thegeneral method has been used previously to produce nanocrystals, severalimportant changes have been made in order to induce one-dimensionalgrowth in the system. To synthesize the nanowires, PbO is first reactedwith oleic acid in the non-coordinating solvent 1-octadecene. Thesoluble Pb oleate produced from this reaction is then warmed to 250° C.under N₂ or other inert gas and a solution of Se dissolved intrioctylphosphine is rapidly injected. The volume of the injected Sereagent solution is important, as it causes an immediate decrease in theoverall temperature of the reaction. If this temperature change is toolarge, the product will be of inferior quality or fail to form entirely.Insufficient heating following the drop will also produce a similareffect. When a molar excess of Pb is present in the reaction (at apreferred ratio of 5:1 Pb:Se) the formation of one-dimensional nanowirestructures is observed as the exclusive product, while lower Pb:Seratios produce mixtures of nanocrystals and nanowires. When bothmorphologies are formed, extraction of the product with a non-polarsolvent such as hexane can be used to isolate the wires by exploitingsolubility differences between the two materials. Addition of stronglybinding secondary ligands to the Pb reagent prior to the introduction ofthe Se reagent improves the surface morphology of the final product. Inthe case of tetradecylphosphonic acid, smooth and uniform wires can beformed as illustrated in FIG. 1. The lengths of the nanowires producedusing this method range from several hundred nanometers up to ˜5 μm,with diameters less than 20 nm. The final product is dispersible inorganic solvents for transfer or post-synthetic manipulation. Thefactors found to be critical for the isolation of the nanowire structurepreferentially over nanocrystals are a high Pb:Se ratio, and maintaininga temperature above 200° C. at all times following addition of the Sereagent. Careful monitoring of the temperature is critical to meetingthis requirement, as is the ability to reproduce the same drop intemperature from reaction to reaction. As long as these requirements aremet, adjustment of the addition temperature can be used to control thediameter of the nanowires, with temperatures lower than 250° C. reducingthe average diameter, while temperatures above 250° C. increase theaverage diameter. Deviation from these procedures leads to isolation ofeither nanocrystals or shorter one-dimensional structures which possessa high degree of surface roughness and are less suitable for electrontransport applications.

Previous methods for the solution preparation of PbSe nanowires havebeen reported in the literature, but are hindered by issues related totheir reproducibility, dimensional control, or product purification asdetailed in the Description of Related Art.

Additionally, all of these procedures are conducted at a relatively highdilution as summarized in Table 1.

TABLE 1 illustrates the overall concentrations of PbSe nanowiresynthesis reactions Concentration (M) Method (based on Pb reagent) PreyMeth 1 0.01-0.05 Prey Meth 2 0.02 Prey Meth 3 0.06 This Disclosure 0.12

The overall concentration of the reaction is an important considerationfor eventual scale up of the procedure, as dilute reactions requireprogressively larger amounts of solvents for both the synthesis as wellas the purification procedure.

The method detailed in this disclosure circumvents these issues throughthe use of a more reproducible, higher concentration, single stepreaction in a non-coordinating solvent and permits isolation ofexclusively one-dimensional PbSe structures with diameters <20 nmsuitable for use in nanoelectronics and photovoltaic applications.

Furthermore, the concentrations and non-coordinating solvent employed inthe method make it suitable for eventual scale-up.

As further detailed herein, this invention concerns a method of makingnanowires involving reacting PbO with oleic acid in 1-octadecene orother non-coordinating solvent and producing Pb oleate in a flask,heating the Pb oleate to between 225 and 275 C under inert gas,injecting a first solution of Se dissolved in trialkylphosphine andproducing a second solution, heating the second solution, maintainingthe temperature >200 C, and resulting in nucleation and growth of PbSenanowires. Excess Pb can be utilized. For example, the ratio of Pb:Secan be between 1:1 and 5:1.

This method can further include the step of adding strongly bindingsecondary ligands to the Pb oleate. The addition of strongly bindingsecondary ligands prior to the addition of the Se results in improvedsurface morphology of the PbSe nanowires.

This method can further include the step of adding tetradecylphosphonicacid. The step of adding tetradecylphosphonic acid results in smooth anduniform nanowires.

This method can produce nanowires having the ability to combine quantumconfinement effects in two-dimensions with a long axis conducive toelectron transport over macroscopic distances. The PbSe nanowires canhave a length of from about several hundred nanometers up to about 5 μm.The PbSe nanowires can have diameters less than about 20 nm.

Furthermore, the PbSe nanowire can have an aspect ratio of about 100:1and have a diameter of less than 20 nm. The PbSe nanowire can have theability to combine quantum confinement effects in two-dimensions with along axis conducive to electron transport over macroscopic distances.

EXAMPLE

A mixture composed of PbO (0.2256 g; 1.0 mmol), oleic acid (0.9660 g;3.4 mmol), TDPA (0.1012 g; 0.36 mmol), and 1-ODE (3.9115 g; 15 mmol) wasprepared in a three neck round bottom flask. The flask was equipped witha magnetic stir bar, thermocouple, vacuum inlet, and heating mantle,then connected to a Schlenk line and evacuated for 20 min (˜0.6 Ton) todegas the reagents. During this time, the temperature of the system (asrecorded by the thermocouple in the solvent) was brought to ˜120° C.Once bubbling of the mixture had ceased, the system was backfilled withN₂ and the stopper replaced with a septum sealed adapter connected to anoil bubbler. The temperature was brought past 150° C. under N₂ purge,resulting in the formation of Pb oleate as indicated by formation of aclear, colorless solution, after which the system was held underpositive N₂ for the remainder of the reaction. Heating was continueduntil the solution reached 250° C. During this time, Se (0.0153 g; 0.19mmol) was added to a separate flask, septum sealed, and purged with N₂.A syringe was used to add 2 mL of TOP to the Se, and the mixturesonicated until the Se had completely dissolved in the TOP. Thissolution was loaded into a syringe and rapidly injected into thereaction flask containing the Pb oleate. An immediate temperature dropto 210° C. occurred, accompanied by a darkening of the solution colorapproximately 10 sec following injection of the reagent. The temperatureof the reaction increased to 230° C. over the course of the 1 minreaction. The heating mantle was then removed and the solution cooledrapidly to <70° C. Under an N₂ purge, 2.5 mL of heptane was added,followed by 2.5 mL of EtOH. The dark mixture was centrifuged (2500 RPM;10 min) to isolate a black solid material. It was separated bydecantation, dispersed in 2.5 mL of heptane, and re-precipitated byaddition of 5 mL of EtOH. The solid was then re-isolated by centrifuge(2500 RPM; 10 min) and washed one additional time with 2 mL of hexane.Following isolation and drying under N₂, 5.8 mg of dark solid wasobtained.

Careful adherence to the procedures described above are required toobtain suitable product in a reproducible fashion. Changes to theligands and solvents have been found to impact the product morphologyconsiderably. PbS or PbTe can be formed through a similar method.

While embodiments of the present disclosure have been shown anddescribed, various modifications may be made without departing from thescope. It is to be understood that even though numerous characteristicsand advantages of the present invention have been set forth in theforegoing description, together with details of the structure andfunction of the invention, the disclosure is illustrative only, andchanges may be made in detail, especially in matters of shape, size andarrangement of parts within the principles of the invention to the fullextent indicated by the broad general meaning of the terms in which theappended claims are expressed.

1. A PbSe nanowire having an aspect ratio of about 100:1 and having adiameter of less than 20 nm.
 2. The PbSe nanowire of claim 1 having theability to combine quantum confinement effects in two-dimensions with along axis conducive to electron transport over macroscopic distances. 3.A PbSe nanowire produced by the process comprising reacting PbO witholeic acid in a non-coordinating solvent and producing Pb oleate in aflask; heating the Pb oleate to between 225° C. and 275° C. under inertgas; injecting a first solution of Se dissolved in trialkylphosphineinto the flask and producing a second solution; heating the secondsolution; maintaining the temperature greater than 200° C. in the flask;and resulting in PbSe nanowires.
 4. The PbSe nanowire of claim 3 whereinthe molar ratio of Pb:Se is between 1:1 and 5:1.
 5. The PbSe nanowire ofclaim 4 wherein PbSe nanowires produced have the ability to combinequantum confinement effects in two-dimensions with a long axis conduciveto electron transport over macroscopic distances.
 6. The PbSe nanowireof claim 5 wherein the PbSe nanowires have a length of from aboutseveral hundred nanometers up to about 5 μm and have diameters less thanabout 20 nm.
 7. The PbSe nanowire of claim 3 wherein thenon-coordinating solvent is 1-octadecene.